A defining characteristic of a wireless channel is the variations of the channel strength over time and over frequency. The variations can be roughly divided into two types as shown in FIG. 1—slow fading (or large-scale fading) and fast fading (or small-scale fading). Slow fading is due to path loss the signal experiences as a function of distance and shadowing by large objects such as buildings or hills, and is typically independent of frequency for the bandwidths of interest. In contrast, fast fading is due to the constructive and destructive interference of the multiple signal paths between the transmitter and the receiver, and is typically frequency dependent.
A related characteristic of the wireless channel is the coherence time, which is a measure of the minimum time required for a magnitude change or phase change of the wireless channel to become decorrelated from its previous value. In the context of the coherence time, the wireless channel is defined as exhibiting slow fading if the coherence time is much longer than the delay requirement (or latency) of the application, and exhibiting fast fading if the coherence time is shorter. The operational significance of this definition is that, in a fast fading channel, coded (or uncoded) symbols can be transmitted over multiple fades of the wireless channel, whereas in a slow fading channel, the symbols experience only a single realization of the wireless channel. Thus, in a fast fading channel, a receiver may exploit the inherent time diversity of the fast fading channel in order to increase the throughput or reliability of the link. However, in the presence of a deep fade (reduction of the channel magnitude to a value less than that required to maintain an acceptable bit error rate), receiver performance is negatively impacted for that duration. The ability to induce fast fading from multiple decorrelated fading processes enables the receiver to improve communication performance.